Hmmm, I have four lyretails and many other fish in my 33 long but my plants take care of the bio-load so I guess some of that will depend on the setup. Lyretails aren't big like balloons. They get up to around 4" max long though like any other molly. Its funny because AQ Advisor only shows me at 80% stocked with my fish (below in my sig).Although 5 lyretail mollies would make a 30g WAY overstocked, in terms of bioload, so he would need an even bigger tank in this case.
Well thats true about the plants! You'd laugh if you saw my 10g with the 50 or so molly fry. Some are about an inch now so I'm going to see about taking them to the local pet store this weekend.